Deep Carbon Cycle Induced By Paleo-asian Oceanic Slab Subduction And Its Contribution To Lithospheric Refertilization Under The North China Craton

Deep carbon cycle regulates the exchange of carbon between the Earth's surface and deep mantle,which is important for modulating Earth's atmospheric climate over the geological timescales,the evolution of life,mantle evolution,and migration of metallogenic elements.Thus,the subduction processes are the contact of carbon between the Earth's surface and the mantle.Among diverse subducted materials,carbon is hosted by sedimentary carbonate rocks,carbonated sediments,carbonated basalt and organic matter.There are now abundant studies on the subduction processes of carbonated basalt,however,very few studies on carbonated sediments or sedimentary carbonate rocks.In order to provide the details on the mechanisms of transport of carbonated sediments and sedimentary carbonate rocks and their contribution to mantle evolution,we present the studies on the carbonatites intruding Neogene alkali basalts in the Hannuoba,the peridotite xenoliths in the Oligocene Fanshi basalts and the spinel-facies mantle peridotites and pyroxenites from the Balmuccia and Baldissero peridotite massifs of the Ivrea Zone in the Italian Alps.The Hannuoba carbonatites are mainly composed of calcite and medium-to coarse-grained silicate macrocrysts set in a matrix of fine-gained calcite,and contains moissanite,highly disordered graphite and zircon.Trace element patterns with positive Sr and U anomalies,negative HFSE?Nb,Ta,Zr,Hf and Ti?and Ce anomalies,high ⁸⁷Sr/⁸⁶Sr ratios?0.70522-0.70796?and high?¹⁸O_SMOW values?22.2–23‰?indicate that this carbonatite had a limestone precursor.However,the presence of coarse-grained mantle-derived clinopyroxene,orthopyroxene and olivine,and chemical features of the carbonates suggest that the carbonate melts were derived from mantle.The carbonates have high¹⁴³Nd/¹⁴⁴Nd ratios?0.51282-0.51298?and show negative correlation between CaO and Ni contents,suggesting reaction between carbonate melt and peridotite.These features suggest that the carbonatites could be derived from subducted limetione reacting with the mantle peridotites.Zircons from the Hannuoba carbonatites show a wide range of ages ranging from Precambrian to Phanerozoic.The age spectra of the Precambrian zircons in the carbonatites exhibit prominent age peaks at²500 Ma,1800-1900 Ma,1313-1392Ma,1122-1135 Ma,908-937 Ma,734-807 Ma,and 623-628 Ma.The age spectra is consistent with those of the southern Central Asian Orogenic belt?CAOB?,but is not compatible with a northern NCC lithosphere source.Furthermore,the 300-400 Ma Phanerozoic zircons show positive Hf isotopic compositions(?Hf_?t?=7.7-13.5)similar to those of the southern CAOB,but contrasting strongly with those from the northern NCC.All these features suggest that the subducted limestone precursor for the Hannuoba carbonatites was derived from the Paleo-Asian Ocean.To understand better the melting behavior of subducted limestone and its interaction with the lithospheric mantle,we conducted the melting relations of a natural limestone in contact with natural dunite at 2-3 GPa and 900-1250°C.The results show that carbonatite melt can form at temperatures as low as 950-1050°C and 2-3 GPa during interaction between limestone and olivine.We find that reaction with olivine strongly depresses the solidus for subducted limestone by550°C lower than the solidus of pure calcite.The solidus of limestone in contact with mantle could be higher than the subduction zone,implying that carbonates will not be mobilized in the subduction zone.Due to low density and viscosity,large scale carbonates could be transferred to the mantle through detaching from the downgoing slab to form buoyant diapirs.Melting of limestone could happen during its migration towards the overlying mantle wedge owing to lower solidus than mantle geothermal.Experimental carbonatite melt has trace element contents similar to the starting limestone,suggesting limited trace element fractionation during melting of limestone,consistent with the conslucion for the Hannuoba carbonatite with geochemical features of recycled limestone.Subduction of carbonated materials by the Paleo-Asian Ocenaic slab could trigger the extensive lithosphere refertilization under the northern margin of the NCC.Here we use in-situ major and trace element and Sr isotopic compositions and mineral Sr-Nd isotopes of the peridotite xenoliths?lherzolites with minor harzburgites?in the Oligocene Fanshi basalts from the northern North China Craton?NCC?to characterize and decode two metasomatic events caused by subduction of carbonated sediments.Clinopyroxenes?Cpx?in the harzburgites are characterized by high?La/Yb?_N?8.4-66?and Zr/Hf?30-66?ratios but low Ti/Eu ratios?478-1268?,suggesting that they have been metasomatized by carbonatitic melt.Highly enriched Sr-Nd isotopic compositions of these Cpx(⁸⁷Sr/⁸⁶Sr=0.70640-0.70716,¹⁴³Nd/¹⁴⁴Nd=0.512304)imply that metasomatic carbonatitic melt was derived from carbonated sediments.Cpx in the lherzolites display lower?La/Yb?_N?0.16-10.6?and Zr/Hf ratios?25-39?,suggesting that the later metasomatic agent is probably a carbonate-rich silicate melt and not carbonatite.Sr and Nd isotopes of these Cpx(⁸⁷Sr/⁸⁶Sr=0.702075-0.706148,¹⁴³Nd/¹⁴⁴Nd=0.512410-0.513286)exhibit negative correlation along a simple mixing line between depleted mantle and carbonated sediments.The timing of the two metasomatic events and the regional tectonic setting are consistent with derivation of the carbonated sediments from the Paleo-Asian Oceanic slab,and both contributed significantly to lithospheric refertilization under the northern NCC.In order to better identify possible contributions of subducted carbonate-bearing materials from the Pale-Asian oceanic slab to lithospheric modification under the NCC,we present Ca isotope data for peridotite xenoliths from Fanshi,northern North China Craton.The?^44/40Ca values of clinopyroxenes?Cpx?in the Fanshi peridotites varies from+0.84‰to+1.17‰and?^44/40Ca of orthopyroxenes?Opx?from+0.82‰to+1.22‰.A harzburgite,modified only by the early carbonatite metasomatism,has the highest?^44/40Ca?+1.17‰?.The positive trends between?^44/40Ca and ⁸⁷Sr/⁸⁶Sr,?La/Yb?_N in the clinopyroxenes of the lherzolites indicate that the second metasomatism also resulted in high?^44/40Ca in the clinopyroxenes of the lherzolites?up to+1.09‰?.The heavy Ca isotope signature in these metasomatic clinopyroxenes applies to both first carbonatite melt and second carbonate-rich silicate melt derived from carbonated sediments?>+1.17‰and⁺1.09‰,respectively?.However,heavy Ca isotope composition is inconsistent with low?^44/40Ca in carbonated sediments from the Paleo-Asian ocean,but could be the characteristic of an evolved sediment-derived hydrous carbonate melt.This implies significant Ca isotope fractionation during subduction of carbonated sediments.Understanding Ca isotope fractionation during magmatic processes in the upper mantle is prerequisite for tracing recycled carbonate in the mantle using Ca isotopes.We have performed precise and accurate Ca isotopic analyses on a series of well-characterized spinel-facies mantle peridotites?lherzolite,harzburgite and dunite,n=29?,pyroxenites?websterite,clinopyroxenite and orthopyroxenite,n=15?from the Balmuccia and Baldissero peridotite massifs of the Ivrea Zone in the Italian Alps.These peridotites underwent variable degrees of melting and melt-peridotite reaction,whereas the pyroxenites are mainly the products of melt-peridotite reaction and crystallization of migrating basic melts from the asthenosphere.The lherzolites from Balmuccia and Baldissero show?^44/40Ca values of 0.94±0.11‰?2sd,n=22?,which are uniform within long-term external reproducibility?±0.14‰,2sd?.?^44/40Ca of the harzburgites?0.83‰to0.92‰?do not differ from those of lherzolites,including those with a history of intensive melt-peridotite reaction to form replacive dunites.The websterites and spinel clinopyroxenites display?^44/40Ca of 0.86±0.10‰?n=14?,within the range of the lherzolites and harzburgites.The indistinguishable?^44/40Ca among these very diverse mantle rocks is the consequence of the overwhelming control of stable Ca isotopes by clinopyroxene in the magmatic processes involved,because it dominates the budget of Ca?>90%for harzburgites,lherzolites,websterites and clinopyroxenites?.Only the clinopyroxene-poor?<3 wt.%?dunites and orthopyroxenite show higher?^44/40Ca?e.g.,1.11‰to 1.81‰and 1.13‰,respectively?.This reflects the signatures of olivine and orthopyroxene which display higher?^44/40Ca than clinopyroxene.These results and modeling suggest that negligible Ca isotope fractionation?<0.12‰?occurs during<25%of partial melting,silicate melt-peridotite reaction,or magmatic differentiation in the upper mantle.This study reveals the mechanism of transport of carbonate materials from the surface to mantle depths and back to the Earth's surface via fluid and melt transport,the contribution to cartonic lithospheric refertilization from carbonate recycling and the application of Ca isotopes on mantle evolution and deep carbon cycle.The results in this thesis could provide some important observational data to deep carbon cycle and could expand the application prospects of Ca isotopes on mantle evolution and deep carbon cycle.